(1) Field of the Invention
The present invention concerns an orthogonal phase modulation method and its demodulation method. In further detail, it concerns a method of modulating and transmitting or storing in an arbitrary system such as on a magnetic recording medium two color signals or two separate luminance signals which will not extend the overall width of its occupied band, which is free of data missing and AM noise effects, and a demodulation method therefor. Accordingly, the present invention is most useful not only for transmission of color picture image signals but also for obtaining high quality pictures in an arbitrary storage of picture image data, such as video recording of moving or still pictures.
(2) Description of the Prior Art
In video recording using magnetic tapes and discs of the prior art, luminance signals were frequency modulated (FM), and two color signals were either amplitude modulated or alternately (line sequentially) FM modulated by 1 horizontal scanning period (1H), and these FM luminance signals and two AM color signals or line sequential FM color signals were recorded by multiplexing on a magnetic medium. In recent years, a method of recording luminance signals by phase modulating the same (PM modulation recording system) was disclosed in Japanese Patent Publication No. 56-51406 and Japanese Patent Application Lay-open Print No. 53-41126.
The PM modulation system mentioned above is intended for high density recording of movie pictures (moving picture images), and is advantageous in that demodulation is carried out without cross talk despite the high density recording without guard band or by partial overwriting at the time of recording, or tracking errors at the time of reproducing. This method is outlined in the following paragraphs (1) to (5).
(1) The carrier to be modulated by the luminance signals is synchronized with the relative movement between the magnetic medium and the magnetic head,
(2) the modulation index mp is controlled to be 1.3 radian or less and the carrier is PM modulated by luminance signals, and
(3) these PM luminance signals are recorded in such a way that the positions of the vertical and horizontal synchronizing signals in the adjacent tracks become aligned with each other in a direction at right angles to the length of said track and such that the carrier phases in the adjacent tracks will also become aligned.
When PM modulation recording is thus performed,
(4) the carrier component amplitudes recorded in respective tracks are substantially the same because mp.ltoreq.1.3 and the carrier components in the reproduced signals become constant even though the magnetic head overrides the adjacent tracks because the components are of the same phase among tracks.
(5) Because mp.ltoreq.1.3, components of more than secondary side bands may be disregarded; and because synchronous signals are aligned between tracks, the side band components of the regenerated signals merely become the composite of plural frames or fields even when the magnetic head overrides the adjacent tracks at the time of reproduction. Therefore, there arises no difficulty for the movie since it is a composite of strongly correlated picture images. On the other hand, the FM recording system discussed above employs the so-called tilted azimath system where azimath is varied by the adjacent tracks for a high density recording.
As described above, various methods are employed in the prior art for high density recording, but they were insufficient in the picture quality particularly in video recording of still picture images.
For high quality video recording, wide band zones should be respectively allotted a luminance signal (Y) and two color signals (C) for modulation. However, there are two kinds of color signals, for example two color difference signals R-Y and B-Y, which must be detected independently in regeneration, thereby creating various difficulties.
We shall now explain representative examples (i), (ii) and (iii) of the color signal modulation method.
(i) A recording method of allotting to two color difference signals, R-Y and B-Y, zones which are separable from each other as shown in FIG. 1(a), and FM modulating the same. Because of the upper limit in frequency characteristics of a magnetic record, when independent exclusive zones for modulation are allotted to the luminance signal and two color signals, the zone occupied by respective signals in the overall frequency zone necessarily becomes narrow, thus deteriorating the picture quality.
(ii) A recording method of FM modulation which is also called the line sequential recording system wherein color difference signals of R-Y and B-Y are alternately switched by 1 H to be allotted to one exclusive zone. Although said zones of each signal become larger within the limited area, the picture quality also deteriorates because both R-Y and B-Y miss 1/2 of the color data.
(iii) A recording method by balanced modulation (BM) which is a kind of AM modulation to BM modulate the two carriers having the same frequency but phases different by 90 degrees by the color difference signals of R-Y and B-Y. The zones occupied by each of the color signals are large because two color signals are allotted to the same zone, and there are no color data missing. However, because it is an AM system, the picture quality becomes inferior due to AM noises caused by changes of the touch of a magnetic head in recording/reproducing. This inferior picture quality presents problems in visual characteristics when recording/transmitting still picture images.